A high-voltage battery that supplies electric power to a travel drive system including a motor and a low-voltage battery that supplies electric power to in-vehicle apparatus such as an air conditioner and an audio device are mounted in an electric vehicle and a hybrid car that have been popular recently. Each battery is configured with a secondary battery that can be charged and discharged. For example, a lithium ion battery is used as the high-voltage battery and a lead battery is used as the low-voltage battery. A bidirectional DC-DC converter is provided between the high-voltage battery and the low-voltage battery.
The bidirectional DC-DC converter has a voltage boosting function and a voltage drop function. For example, in a case where the remaining amount of the low-voltage battery is not sufficient, a voltage of the high-voltage battery is dropped by the bidirectional DC-DC converter and supplied to the low-voltage battery to charge the low-voltage battery. In addition, in a case where the remaining amount of the high-voltage battery is not sufficient, a voltage of the low-voltage battery is boosted by the bidirectional DC-DC converter and supplied to the high-voltage battery to charge the high-voltage battery. As such, by using the bidirectional DC-DC converter, electric powers of the two different DC power can be mutually complemented.
There are two types of bidirectional DC-DC converters: an insulation type and a non-insulation type. In the bidirectional DC-DC converter of the insulation type, for example, as described in Japanese Unexamined Patent Application Publication No. 2015-228788, the low-voltage battery side and the high-voltage battery side are insulated by a transformer, and switching elements are respectively provided on a primary side and a secondary side of the transformer. In the bidirectional DC-DC converter of the non-insulation type, for example, as described in Japanese Unexamined Patent Application Publication No. 2003-304644, the low-voltage battery side and the high-voltage battery side are not insulated from each other, and a voltage-drop switching element and a voltage-boosting switching element are provided on an electric path connecting the respective batteries.
FIG. 10 illustrates an example of a bidirectional DC-DC converter of a non-insulation type of related art. A bidirectional DC-DC converter 51 is provided between a high-voltage battery E1 and a low-voltage battery E2, and includes a voltage-drop switching element Q11, a voltage-boosting switching element Q12, a smoothing capacitor C, an inductor L, and a switch SW. D11 and D12 are parasitic diodes of switching elements Q11 and Q12, respectively.
In a case where charging is performed from the high-voltage battery E1 to the low-voltage battery E2, in a state where the switch SW is turned on, a voltage-boosting switching element Q12 is turned off and a voltage-drop switching element Q11 is turned on and off by a PWM signal with a predetermined duty. Thereby, a voltage of the high-voltage battery E1 is dropped in accordance with the duty of the PWM signal and is supplied to a low voltage side to charge the low-voltage battery E2. In addition, in a case where charging is performed from the low-voltage battery E2 to the high-voltage battery E1, in a state where the switch SW is turned on, the switching element Q11 is turned off and the switching element Q12 is turned on and off by the PWM signal. During a period when the switching element Q12 is turned on, electric energy is accumulated in the inductor L. During a period when the switching element Q12 is turned off, the electric energy of the inductor L is discharged via the diode D11, and the high-voltage battery E1 is charged by the boosted voltage.
In the DC-DC converter 51 illustrated in FIG. 10, if the switch SW is turned on at the time of starting up, an excessive inrush current flows from the high-voltage battery E1 to the smoothing capacitor C via the switch SW, and thereby, the smoothing capacitor C may be destroyed. Therefore, in order to suppress the inrush current, preliminary charging the smoothing capacitor C is performed in the related art, before turning on the switch SW (Japanese Unexamined Patent Application Publication No. 2015-228788, Japanese Unexamined Patent Application Publication No. 2007-295699, Japanese Unexamined Patent Application Publication No. 2007-318849, and Japanese Unexamined Patent Application Publication No. 2009-232502).
However, in the bidirectional DC-DC converter 51 of the non-insulation type illustrated in FIG. 10, a low voltage side and a high voltage side are not insulated unlike the bidirectional DC-DC converter of the insulation type, and thereby, the low-voltage battery E2 is constantly connected to the high voltage side via the parasitic diode D11 of the switching element Q11. Accordingly, even if the switch SW and the switching elements Q11 and Q12 are all turned off, a current flows from the low-voltage battery E2 to a load (not illustrated) on the high voltage side via the parasitic diode D11 of the switching element Q11, and thereby, the load abnormally operates or the low-voltage battery E2 is consumed.
As a countermeasure against this, a DC-DC converter 52 illustrated in FIG. 11 includes a switching element Q13 for preventing a reverse current from flowing from a low voltage side to a high voltage side, which is provided between the low-voltage battery E2 and the inductor L. D13 is a parasitic diode of the switching element Q13. Since a direction of the parasitic diode D13 is opposite to a direction of the low-voltage battery E2, a current does not flow from the low-voltage battery E2 to the high voltage side in a state where the switching element Q13 is turned off.
However, even if the switching element Q13 is turned on before the switch SW is turned on to preliminarily charge the smoothing capacitor C from the low-voltage battery E2 when the DC-DC converter 52 starts up, an inrush current flows from the low-voltage battery E2 to the smoothing capacitor C via the parasitic diode D11 by turning on the switching element Q13, and thereby, there is a concern that the smoothing capacitor C is destroyed. Accordingly, it is necessary to separately provide a circuit for preliminarily charging the smoothing capacitor C.